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07-05-2023, 03:53 PM
Hi Champ81
You say you have 27mA per tube. What is the plate voltage in this amp at that current? With EL-84 rated at 12W Va should be 444V or less. In post-42 or so, you referred to EL-34, which is fine at 12W since it is rated at 25W max.
Please STOP using signal to set idle.
It is only of benefit for learning and satisfying curiosity to monitor current with a signal present. Obviously, you can become easily confused by the numbers you see. You have to remember that "heat" is what watts means, and this is a quantity that is averaged over time. With an audio signal, and even with a steady sine wave, the instantaneous power / heat can be way higher than the idle heat or even the maximum rated heat, but there are also points in the signal cycle where the heat is far lower or even zero.
Tube ratings are "average". This means that the 25W plate rating for the tube is satisfied if, for example, the voltage across the tube is 500V and the current through it is 500mA, and also if you pulse the current to 100mA at 500V half the time, and the current is zero half the time.
The current rating of 100mA you referred to is likely for a specific application; it is certainly not the cathode current rating for the tube as listed by the manufacturer. An EL-34 cathode can emit substantially higher current than that. You should refer to TUT6 about this.
Back to the cathode biasing... The 50R resistors in your circuit seem a bit low in value (no doubt they are perfect as 50R resistors go
). The implication would be that a single tube could be biased with 200R, which is not the case with EL-34. As stated previously, the single-tube average Rk value is 470-500R, call it 500R to be safe. Two tubes sharing Rk can therefore use 250R and 4-tubes 125R. The total Rk value with the adjustment scheme MUST be higher than what would be typical without the adjustment, otherwise there is no room for adjustment other than hotter than stock - the opposite of what we want.
About Ck: I forget that some readers do not see the subtleties of what I write, especially when it is being read on a glowing screen. So, to be clear:
Ck only has to withstand the voltage that is across it. Typically this might be Vs/10. In the case of the adjustable circuit, we need a bit more voltage than that, so maybe rate the cap for around Vs/5.
The cap voltage rating and its value have nothing to do with each other.
I simply referred to the fact that going from a 450V cap to a 100V cap should reduce its size AND that if this frees up space it is an opportunity to increase the value of Ck.
Higher-value Ck provides better stability of the idle point BUT you have to go significantly higher than the values you see in any guitar amp. Look at hifi examples, where 1mF is common, although even that is too low for a quad of tubes.
Personally, I never use Ck in places I have self-biased output tubes. Including Ck diminishes the sonic difference between fixed-bias and cathode bias.
You say you have 27mA per tube. What is the plate voltage in this amp at that current? With EL-84 rated at 12W Va should be 444V or less. In post-42 or so, you referred to EL-34, which is fine at 12W since it is rated at 25W max.
Please STOP using signal to set idle.
It is only of benefit for learning and satisfying curiosity to monitor current with a signal present. Obviously, you can become easily confused by the numbers you see. You have to remember that "heat" is what watts means, and this is a quantity that is averaged over time. With an audio signal, and even with a steady sine wave, the instantaneous power / heat can be way higher than the idle heat or even the maximum rated heat, but there are also points in the signal cycle where the heat is far lower or even zero.
Tube ratings are "average". This means that the 25W plate rating for the tube is satisfied if, for example, the voltage across the tube is 500V and the current through it is 500mA, and also if you pulse the current to 100mA at 500V half the time, and the current is zero half the time.
The current rating of 100mA you referred to is likely for a specific application; it is certainly not the cathode current rating for the tube as listed by the manufacturer. An EL-34 cathode can emit substantially higher current than that. You should refer to TUT6 about this.
Back to the cathode biasing... The 50R resistors in your circuit seem a bit low in value (no doubt they are perfect as 50R resistors go
). The implication would be that a single tube could be biased with 200R, which is not the case with EL-34. As stated previously, the single-tube average Rk value is 470-500R, call it 500R to be safe. Two tubes sharing Rk can therefore use 250R and 4-tubes 125R. The total Rk value with the adjustment scheme MUST be higher than what would be typical without the adjustment, otherwise there is no room for adjustment other than hotter than stock - the opposite of what we want.About Ck: I forget that some readers do not see the subtleties of what I write, especially when it is being read on a glowing screen. So, to be clear:
Ck only has to withstand the voltage that is across it. Typically this might be Vs/10. In the case of the adjustable circuit, we need a bit more voltage than that, so maybe rate the cap for around Vs/5.
The cap voltage rating and its value have nothing to do with each other.
I simply referred to the fact that going from a 450V cap to a 100V cap should reduce its size AND that if this frees up space it is an opportunity to increase the value of Ck.
Higher-value Ck provides better stability of the idle point BUT you have to go significantly higher than the values you see in any guitar amp. Look at hifi examples, where 1mF is common, although even that is too low for a quad of tubes.
Personally, I never use Ck in places I have self-biased output tubes. Including Ck diminishes the sonic difference between fixed-bias and cathode bias.
